28 



PHOTOSYNTHESIS 



crease in total solar radiation with increasing elevation. Abbot ^ states 

 that the loss of radiation in passing through the last mile of air is almost 

 as great as the entire loss sustained above Mount Wilson (5,675 feet). 



In their passage through the earth's atmosphere some wave lengths 

 are more absorbed than others. The absorption bands of oxygen, ozone, 

 water vapor and carbon dioxide are chiefly in the infra-red and some in 

 the extreme ultra-violet portions of the spectrum. The solar radiation 

 received on the earth lies almost entirely within the wave lengths 0.29 ^ 

 and 2.5 |-i. The spectral energy curve shows that the maximum intensity 

 is at .47 \i, decreasing to almost zero in the ultra-violet and infra-red 



Fig. 1. — Maximum solar radiation per minute in gram calories per square centimeter 

 at Washington, D. C. I. Solar radiation at normal incidence. II. Solar and 

 sky radiation on a horizontal surface, with clouds near the sun, but not obscur- 

 ing it. III. Solar radiation on a horizontal surface, with cloudless sky. (From 

 Kimball.) 



regions beyond the limits mentioned and exhibiting a number of depres- 

 sions at the Fraunhofer lines and in the regions where the earth's at- 

 mosphere exercises selective absorption. Abbot ' has made extensive 

 studies of the energy of the solar spectrum and by means of the spectro- 

 bolometer has prepared energy curves, called holographs, which show the 

 distribution of solar radiation and the transmission of the atmosphere at 

 all parts of the spectrum. These investigations are of fundamental im- 

 portance for every phase of the question of solar energy. He has also pre- 

 pared a table of the transmission of the atmosphere at various wave 

 lengths. This is reproduced in Table i. "The ^ values represent the frac- 

 tion of intensity of the solar beam outside the atmosphere which would 

 remain in a direct beam transmitted vertically to the earth's surface. Aver- 

 age values for cloudless (but not necessarily hazeless) days are given, as 



•Abbot, The Sun. p. 293. 



' .\bbot, Smithsonian Inst. Ann. Astrophysical Obser., 3, 21 (1913). 



" Abbot, Ibid., 197. 



